Let's say crate B depends on crate A with a pinned dependency, and uses one of its types in a public interface.

Crate C depends on them both. It now can't bring in updates to A until B does, and when B updates that's a breaking change, so it better bump its major version.

Take a look at this trick, for example, for foundational crates updating their major version: https://github.com/dtolnay/semver-trick

Now imagine that being an issue every single patch update.